Abatacept is a soluble human fusion protein that selectively modulates the costimulatory signal required for full T-cell activation. It is approved for the treatment of moderately to severely active rheumatoid arthritis. It is also approved for the treatment of moderately to severely active polyarticular juvenile idiopathic arthritis in children 6 years of age or older. Abatacept is a lyophilized powder administered as a 30 minute intravenous infusion. Dosage, as in rheumatoid arthritis, is weight based and is fixed throughout the course of treatment. Abatacept or placebo will be administered as a 30 minute intravenous infusion at baseline, weeks 2, 4, and every 4 weeks for 5 cycles (weeks 8, 12, 16, 20) for a total treatment period of 6 months. There will be a 6 month observational period following the treatment period.